1. Field of the Invention
The present invention generally relates to an optical calorimeter system and, more particularly, to an optical calorimeter system for executing a real and short time calorimetric measurement of optical power.
2. Description of the Related Art
In measurement of optical power such as laser beam, calorimetric measurement executed by using an optical calorimeter is known as a method of executing very precise measurement corresponding to a primary standard level of a national agency.
The calorimetric measurement is a technique in which optical power is substituted into the calories and then direct current substitution is executed to measure the optical power.
In the calorimetric measurement as described above, however, a complicated operation is generally required, and a long time up to about one hour is necessary. That is, in the former systems, a high-precision optical power meter becomes expensive, and an expert skilled in the measurement is required, resulting in disadvantages for a user. Also, an error factor is increased, so that precise measurement can not easily executed.
A cause for such problems in the calorimetric measurement will be described below.
FIG. 5 shows an arrangement of calorimetric measurement executed by using a general calorimeter. In this arrangement, a temperature reference jacket 1 incorporates an absorber 11 having a heater 11a, a cooler 12, and a temperature sensor 13. The jacket 1 and the absorber 11 are connected by a thermal circuit via the cooler 12 and by a thermal circuit via the sensor 13. The absorber 11 converts input optical power into calories. Reference numeral 100 denotes a controller for controlling the heater 11a and the cooler 12 in accordance with an output from the sensor 13.
Referring to FIG. 5, while input light L is off, the controller 100 controls the cooler 12 and the heater 11a by a control amount slightly higher than the power of the input light L. Thereafter, the controller 100 PID-controls (proportional, integral and differential controls) the heater 11a on the basis of temperature difference data from the sensor 13 while maintaining the control amount for the cooler 12 at a predetermined value, thereby obtaining a temperature equilibrium between the absorber 11 and the jacket 1. The controller 100 records a first control amount P.sub.h1 used to control the heater 11a when the temperature equilibrium is obtained.
The input light L is then input, and the controller 100 PID-controls the heater 11a on the basis of the temperature difference data from the sensor 13 while maintaining the control amount for the cooler 12, thereby obtaining a temperature equilibrium between the absorber 11 and the jacket 1. The controller 100 records a second control amount P.sub.h2 used to control the heater 11a when the temperature equilibrium is obtained.
The controller 100 calculates power P.sub.i of the light input to the absorber 11 on the basis of the first and second control amounts by using the following equation: EQU P.sub.i =E(P.sub.h1 -P.sub.h2)+P.sub.r ( 1)
where E is the ratio of the optical power input to the absorber 11 to DC power consumed by the heater 11a so as to be equal to a temperature given by the optical power. This ratio is measured and known in advance.
P.sub.r is the optical power value not perfectly absorbed but reflected by the absorber. P.sub.r is an error factor and must be designed to be small. P.sub.r will be neglected in the following description.
To precisely the optical power, it is necessary to carry out a calorimetric measurement. That is to say, it is required to obtain a temperature equilibrium (temperature equalized) state upon PID control executed by the controller 100. This is a cause of a long measurement time (about one hour for each measurement). That is, unbalance is present in initial values set for the cooler 12 and the heater 11a by the controller 100, and the controller 100 performs PID control at a higher response speed than the time constant of the temperature difference sensor 13 for detecting a temperature difference between the cooler and the heater thereafter. Therefore, a very long time period is required until a ringing phenomenon in which overshooting and undershooting are repeated in temperature difference data between the cooler and the heater occurs to eliminate the temperature difference (actually, the temperature difference is converged within a predetermined allowable difference range).
A demand has arisen, therefore, for a system capable of reducing the measurement time and facilitating an operation.
A conventional calorimetric measurement system capable of measuring precise the optical power is disclosed as "Automatic Calibration Systems for Laser Power Standard (I) and (II)" in "Bulletin of the Electrotechnical Laboratory", Vol. 50, Nos. 4 and 7.
In the conventional calibration system (I), laser power is measured by a calorimeter as the primary standard. Then, an object power measurement equipment is set substitute the calorimeter and output optical power of measurement is executed. Calibration coefficients are determined from the ratio of optical power measurement results as above described. However, in this system (I), there is a problem that a direct error occurs in stability of laser power, during the long time (about one hour) which is spent for the measurement, thereby reducing the measurement time.
In this system (II) so as to use a secondary standard, the operation is performed in order to obtain the effective efficiency of a thermopile (corresponding to the absorber 11 described above) by the calorimetric measurement, and execute a comparison measurement by a thermo electric power only. That is, system (II) has a merit that the comparison measurement can be executed in a short time, but has a demerit that an error occurs since the calibration of optical power is executed without using the calorimetric measurement.
The present invention intends to provide a method of measuring optical power by using a calorimeter, in which all measurements are executed by calorimetric measurements without executing any preliminary measurement as described above, thereby reducing a measurement time, and an apparatus therefor.